In many applications, instead of unidirectionally acting controls, feedback control methods are being used with increasing frequency, e.g. in the automotive sector, as a control method for safety-relevant functions in particular (transmission, clutch, ABS, airbags etc.) or for comfort functions (automatic windows, sunroofs, automatic stopping systems etc.). In one control mode, by means of a feedback control circuit consisting of a control system that influences the control mode and a controlled system that effects the feedback, the actual value of a controlled variable (the physical variable to be controlled) is compared with the specified setpoint of the controlled variable, and a difference between the setpoint and the actual value (the deviation or the system deviation as a negative deviation) is minimized by means of a manipulated variable that acts on the controlled variable. The setpoint of the controlled variable (the specified setpoint) can be varied by means of a reference variable that is fed to the closed-loop control system.
Depending on the desired application of the control mode, the control system can be realized by means of various controllers which, when there is a system deviation or control deviation, affect one of a number of different control modes. For example, there are proportional controllers, integral controllers, differential controllers, linear controllers or step switches. To reduce the load on the control system, the control system can be constructed from a plurality of controllers that are normally realized in the form of step switches and are activated and deactivated by switching processes, for example. One disadvantage of such systems is the restricted range of action of the various regulators (in particular when step switches are used), i.e. the transition between the control ranges of the controller is not clearly defined. The result is that there are disruptions in the control or the control mode (occurrence of dead areas, overlaps, hysteresis, undefined conditions etc.) and EMC problems that require expensive components to reduce.
The object of the invention is to create a simple control circuit as described in the introduction to claim 1, in which these disadvantages are eliminated, and which has characteristics that are advantageous compared to the prior art.
The invention teaches that this object can be accomplished by the features disclosed in the characterizing portion of claim 1.
In the control circuit taught by the invention, the maximum value of the reference variables that determine the setpoint of the controlled variable (the setpoint) is determined on the basis of the control limits of the individual cascaded controllers of the control circuit connected in parallel, by determining the maximum value of the reference variable by the sum of the control limits of the controller of the control circuit. The reference variable is fed differently to the individual controllers of the control system, so that the controllers of the control [system] circuit arc transmitted by different setpoints of the controlled variable, or different control deviations generated on the basis of the setpoint and the actual value. The reference variable is used as the setpoint for the formation of the control deviation for a first controller, and for the additional controllers, the control deviation of the respective preceding controller is used, i.e. for the second controller the control deviation of the first controller, for the third controller the control deviation of the second controller, etc. in this manner, as the setpoint for the second controller, the reference variable minus the actual value of the controller variable of the first controller is used. For the setpoint of the third controller, the reference variable minus the actual values of the controlled variable of the first controller minus the actual value of the controlled variable of the second controller is used, etc. Accordingly, the resulting control, deviation which is transmitted to the respective controller of the control system, is the difference between this specified setpoint and the actual value of the controlled variable of the respective controller.
The cascaded controllers connected in parallel of the control system are successively activated on the basis of this specification of different setpoints or the control deviations thereby formed.
When a reference value increases from zero (setpoint for the controlled variable), first only the first controller is activated; the second controller is deactivated, because for it, as the setpoint, the control deviation (reduced to zero) of the first controller is specified. Likewise, the additional controllers, if any, are deactivated, because a control deviation occurs in the second controller and in none of the subsequent controllers. The first controller controls the controlled variable according to the specification by the reference variable by itself, until its control limit has been reached (i.e. until it is deactivated or xe2x80x9csaturatedxe2x80x9d). In the event of a further increase of the reference variable (the setpoint for the controlled variable), the first controller can no longer respond to this setpoint command (i.e. it can no longer control the controlled variable), and thus a control deviation occurs. The second controller which has just received this setpoint in the form of a control deviation is therefore xe2x80x9cactivatedxe2x80x9d and takes over the portion of the controlled variable that can no longer be controlled by the first controller (the additional controllers present, if any, are then deactivated, because the control deviation of the second controller is zero). In the event of a further increase of the reference variable (the setpoint for the controlled variable), the second controller ultimately also reaches its control limit. As a result of the control deviation on the second controller that also occurs (which corresponds to the setpoint command for the formation of the control deviation for the third controller), the third controller is consequently activated, and takes over the portion of the controlled variable that can no longer be controlled by the second controller. In the event of a ether increase in the reference variable (the setpoint command for the controlled variable), the additional controllers of the control circuit are activated successively, and in an analogous manner take over the portion of the controlled variable that can no longer be controlled by the preceding controllers (for the final controller of the control system, the setpoint confound for the generation of its control deviation is the control deviation of the penultimate controller). In this manner, the controlled variable can be controlled continuously up to its maximum value determined by the maximum value of the reference variable, because the control limit of the final controller of the control circuit is reached at the maximum value of the controlled variable.
To specify the different setpoints, and thus to generate the different control deviations for the various controllers of the control circuit, a comparison stage with a plurality of comparison elements can be provided. Each controller is thereby associated with a comparison element to generate the respective control deviation, to which the actual value of the controlled variable of the respective controller and the different specified setpoint are fed.
The controllers of the control circuit can preferably be realized in the form or linear controllers which have a continuous control mode up to their control limit.
The advantages of the control circuit are that;
Any desired controllers with different control limits and different control responses can be used as the controllers of the control circuit.
As a result of the setpoint specification of the additional controllers of the control circuit, which can be selected as the control deviation of the respective preceding controllers the control is totally independent of tolerances of components of the control circuit, the dimensions of the controlled circuit and the value of the power supply voltage of the control circuit.
Only one of the controllers of the control circuit is operated in a controlling operation at a time (the other controllers are deactivated or turned off), so that the control circuit has a low power consumption.
The transition between the various control ranges is continuous. Consequently, there are no switching processes, nor any interference with the control process caused by switching processes (occurrence of dead zones, overlaps, hysteresis, undefined conditions etc.).
EMC (electromagnetic compatibility) problems are largely eliminated, so that expensive components are not required to reduce the problems of EMO.